1. Field of the Invention
This invention relates to the separation of materials, produced for example by a mining operation, into fractions of different densities.
2. Description of the Prior Art
In for example the mining of coal, a mixture of coal and shale is produced from the face being mined, and it is necessary to separate the coal from the shale by the use of automatic machinery. Conventionally used for this operation is a wash box (hereinafter referred to as "a wash box of the kind specified") essentially in the form of a vessel divided vertically into a stratification compartment and a reject compartment, with a perforate grid plate extending across upper parts of the compartments. The vessel is filled to a level above the grid plate with a liquid, such as water, and the raw material is deposited on the grid plate at one side, flow of water across the grid plate tending to carry the material across the grid plate.
Means is provided to cause generally vertical pulsations in the liquid in the two compartments, disturbing the raw material on the grid plate, causing the heavy material (the shale) to sink to the bottom adjacent to the grid plate, and the lighter material (coal) to rise to the top. Conventionally, the means to cause the pulsations is provided by a conduit, opening into both the stratification and reject compartments, along which air under pressure is delivered intermittently, the delivery of the pulses to the stratification compartment coinciding with the delivery of pulses to the reject compartment. Thus, as the raw material moves across the grid plate, and in particular across that part of the grid plate extending over the stratification compartment, the raw material is stratified. The heavier material adjacent to the grid plate passes beneath a gate or sill located a short distance above the grid plate, onto that part of the grid plate which extends over the reject compartment, whilst the lighter fraction passes over the gate and from the wash box through a primary outlet. The pulsations applied to the reject compartment lift the material on the grid plate, and cause it to pass over a wall or wier from the reject compartment into an adjacent reject extract chamber.
In such wash boxes, it is necessary to control the thickness of the layer of the heavier material on the grid plate in the stratification compartment so as to ensure reliable separation of materials. If the layer of the heavy material becomes too thick, particles of the heavier fraction will pass out with the lighter material over the gate or sill. Conversely, if the layer of the heavier material is too thin, particles of the lighter fraction will be discharged into the reject extraction chamber with the heavier material.
Thus, it has been proposed in a wash box of the kind specified to sense the thickness of the layer of the heavier material in the stratification compartment and, when a reduction in the thickness of this layer is required, to increase the amplitude of the pulsations being applied to the reject compartment so that the particles of heavier material will pass over the wier and fall into the reject extraction chamber at a greater rate. Conversely, when the layer of the heavier material in the stratification compartment falls below a predetermined level, the amplitude of pulsations applied to the reject compartment may be decreased, reducing the rate at which the particles of heavier material pass over the wier.
In all previous constructions, such control of the amplitude of the pulsations being applied to the reject compartment has been carried out by the use of valves, which are adjustable to vary the effect of the intermittent delivery of air pressure pulses.
However, in all previous constructions of wash box of this kind, the frequency at which pulsations are applied to the reject compartment is equal to the frequency at which the pulsations are applied to the adjacent stratification compartment, albeit the amplitude of the former pulsations may be varied between zero and a maximum.
One conventional method of measuring the thickness of the layer of heavier material on the grid plate is by the use of a float which rests on the top surface of the heavier material, and the position of the float has been used to control the pulsations applied to the reject compartment.
Alternatively, since the pressure which is generated in the stratification compartment will be dependant upon the weight of raw material resting on the grid plate, proposals have been made to connect a vertical tube to the stratification compartment from beneath the grid plate, into which liquid from beneath the grid plate flows, and to sense the mean or average position of the surface of the liquid within this tube by the use of mechanical means which includes a float in the tube, thereby providing an indication of the thickness of the layer of the heavier material. Thus, the position of the float may be used to control the amplitude of the pulsations applied to the reject compartment.
Alternatively, electrodes within such a tube have been used to sense when the level of the liquid within the tube rises to a Predetermined point (corresponding to an undesirably high thickness of heavier material on the grid plate) to cause an increase in the amplitude of the pulsations applied to the reject compartment, or to sense when the level of liquid within the tube falls below a predetermined point (corresponding to an undesirably small thickness of the heavier material on the grid plate) to cause a decrease in the amplitude of the pulsations applied to the reject compartment.
When coal was extracted manually from the face being worked, the raw material would contain a relatively small proportion of reject materials of a heavier density, for example, ten per cent of the raw material would be shale. However, even with the use of coal cutting machinery, until recently coal seams which have been worked have been relatively thick, and of a reliable nature. Thus, it was possible to use the coal cutting machine in a manner such that the proportion of reject materials of heavier density cut from the face with the coal was still relatively small, although usually higher than the figure which was obtained with manual extraction.
However, there is in present times a tendancy to work seams of decreasing reliability, involving at times the cutting of predominantly reject material, and to work seams of shallower depth, and/or over and undercutting to an extent which ensures that all the coal is removed, despite the increase in reject material which will necessarily be produced at the same time.
Thus, not only is the reject content of the raw material becoming increasingly higher (often up to seventy per cent) but also the proportion of reject material may vary considerably over relatively short periods of time.
Present techniques for controlling the operation of wash boxes of the kind specified have heretofore become insufficiently accurate, and have produced difficulty in ensuring that a minimum of reject material is included in the coal, and in ensuring that a minimum of coal is included in the reject material.